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The India-Asia collision is an outstanding smoking gun in the study of continental collision dynamics. How and when the continental collision occurred remains a long-standing controversy. Here we present two new paleomagnetic data sets from rocks deposited on the distal part of the Indian passive margin, which indicate that the Tethyan Himalaya terrane was situated at a paleolatitude of ∼19.4°S at ∼75 Ma and moved rapidly northward to reach a paleolatitude of ∼13.7°N at ∼61 Ma. This implies that the Tethyan Himalaya terrane rifted from India after ∼75 Ma, generating the North India Sea. We document a new two-stage continental collision, first at ∼61 Ma between the Lhasa and Tethyan Himalaya terranes, and subsequently at ∼53−48 Ma between the Tethyan Himalaya terrane and India, diachronously closing the North India Sea from west to east. Our scenario matches the history of India-Asia convergence rates and reconciles multiple lines of geologic evidence for the collision. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau accommodated major upper crustal shortening during Indian Plate oceanic and continental lithosphere subduction. Deciphering whether shortening was continuous or episodic, and how it correlates to major geodynamic changes is challenging. Here we apply anisotropy of magnetic susceptibility (AMS), a sensitive synsedimentary strain indicator, to a ~3 km thick magnetostratigraphically dated sedimentary section (69–41.5 Ma) in eastern Tibet. AMS shows "earliest deformation" fabrics from 69–52 Ma, followed by a sudden change to "pencil structure" fabrics with increasing anisotropy degree at ~52 Ma, dating a sudden increased synsedimentary shortening strain. This change coincides with enhanced sedimentation rates and synsedimentary vertical‐axis rotations of the Gonjo Basin, suggesting a causal link to a marked India‐Asia convergence rate deceleration. We show that AMS analysis provides a strong tool to distinguish between climatic and tectonic causes of sedimentological change and is an asset in identifying discrete tectonic pulses in intensely deformed terrane. Plain Language Summary: How the Tibetan Plateau evolved during India‐Asia convergence and collision is notoriously challenging to decipher. Use of sedimentary records to date periods of tectonic activity is a popular approach, yet distinguishing between tectonic versus climate signals in sediment records can be challenging. The anisotropy of magnetic susceptibility (AMS) is an effective and sensitive technique that reveals tectonic stress fields during sedimentation and changes therein, even in weakly deformed clastic sedimentary rocks. We report a detailed record of AMS data from a ~3 km thick section of redbeds from the Gonjo Basin in eastern Tibet. Previous magnetostratigraphy dated deposition from 69 to 41.5 Ma; during this time period marked sedimentation rate increases and simultaneous vertical‐axis rotations were interpreted to reflect shortening pulses. Our new AMS data indicate an increased shortening strain at ~52 Ma, demonstrating that the sedimentation rate changes are tectonic rather than climatic in origin, showing that a pulse in crustal shortening of Tibet occurred simultaneous with a marked ~52 Ma onset of deceleration of India‐Asia convergence. We show that applying a suite of paleomagnetic and rock magnetic techniques, including magnetostratigraphy and sedimentation rate calculation, vertical‐axis rotation analysis, and AMS analysis, provides a powerful tool to differentiate tectonic versus climatic influence on a sediment archive, allowing the precise dating of discrete deformation phases in intensely deformed regions that evolved over long periods of time. Key Points: The AMS of the Gonjo Basin records sedimentary fabrics overprinted by weak shorteningThe AMS of the Gonjo Basin indicates an increased strain related to shortening at ~52 MaAMS with precise age constraint provides a powerful tool to date deformation phases, allowing discrimination between tectonic versus climatic influences on sediments [ABSTRACT FROM AUTHOR]

• Cretaceous oceanic redbeds in southern Tibet retain a primary remanence. • The central Tethyan Himalaya was constrained to a paleolatitude of 16.7°S at 75 Ma. • Greater India had a N-S extension of ∼910 km at its central part. • This Greater India extent provides a crucial constraint on Tethyan reconstruction. Knowledge of the size of Greater India is critical to deciphering the geodynamic processes of the India-Asia collision, as the size of this landmass primarily determines the amount of continental subduction and crustal shortening. Recently, paleomagnetic data revealed the rapid drift of a Himalayan microcontinent (i.e., the Tibetan Himalaya) during ca. 75–61 Ma, which resulted in a triple-stage India-Asia collision scenario. This scenario implies a relatively small Greater India at 75 Ma (Late Cretaceous), but this hypothesis is only based on one high quality Campanian paleomagnetic pole from the eastern part of the Tethyan Himalaya. Here we report comprehensive petrographic, rock magnetic and paleomagnetic studies on Upper Cretaceous oceanic affinity red beds (CORBs) of the Chuangde Formation from the central part of the Tethyan Himalaya at 83.6°E longitude. These CORBs are characterized by multi-component magnetizations carried dominantly by detrital hematite, which retains a primary remanent magnetization. Accordingly, the high temperature magnetization components (585−690°C) were isolated by high-resolution thermal demagnetization. The new paleomagnetic data provide, after applying an inclination shallowing correction, a Campanian paleopole of 43.3°N/258.3°E, which places the central part of the Tethyan Himalaya at a paleolatitude of 16.7° ± 1.8°S at ca. 75 Ma. Comparison of this paleolatitude with the expected paleolatitude of India, using its apparent polar wander path, shows that the N-S extent of central Greater India was ∼910 km during the middle Campanian. Our paleomagnetic data are in agreement with balanced cross section reconstructions and seismologic interpretations of the India-Asia collision zone and provide additional support for the North India Sea hypothesis. The improved estimate of the size of Greater India has several implications for Indian continental crust behavior and results in an updated reconstruction of the India-Asia collision system in the Campanian. [ABSTRACT FROM AUTHOR]

Abstract: A combined paleomagnetic and geochronological study is reported of Paleogene basalt lavas and an intercalated red bed succession, comprising a minimum of 14 basalt flows and 10 red bed horizons in the Tuoyun Basin of the southwest Tian Shan Range, China. Two basalt matrix samples yield 40Ar/ 39Ar isochron ages of 58.5±1.3 Ma (2σ, MSWD=0.9) and 60.4±1.3 Ma (2σ, MSWD=1.7). These compare well with a previously published K–Ar dilution age of 61.7±2.3 Ma for comparable Paleogene basalts and confirm that the younger pulse of magmatism in this basin is represented by both intrusive and extrusive activity. Demagnetization and component analysis identify a stable characteristic remanence (ChRM) with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250–350 °C or 5 mT. Rock magnetic analysis identifies pseudo-single domain magnetite or titanomagnetite as carriers. The stable ChRM passes a fold test; it was probably acquired at the time of lava emplacement. Results from the bulk of the collection imply that paleomagnetic data from the upper and lower (∼115 Ma) basalt series in the Tuoyun Basin are not distinguishable at the 95% significance level and indicate that this tectonic domain remained essentially stationary with respect to the Earth''s spin axis for ∼50 Ma prior to onset of the India/Asia collision in early Eocene times. It is therefore probable that no paleomagnetically detectable crustal shortening occurred in the southwest Tian Shan prior to collision. Paleomagnetic data sets from the Tuoyun Basin also show that little or no paleolatitude difference is present between the Tian Shan and the reference latitude of Eurasia at ∼60 Ma. This supports previous evidence suggesting that central Asian blocks in the vicinity of the Tian Shan are unlikely to have experienced appreciable northward convergence relative to Eurasia since onset of the India/Asia collision and initiation of the Himalaya. [Copyright &y& Elsevier]

• New high-resolution magnetostratigraphic study constrains the Gonjo Basin from 69 to 41.5 Ma. • Rapid SR and rotation periods suggest pulses of Tibetan crustal shortening at 69-67 Ma and 52-48 Ma. • Tibetan crustal shortening pulses correlate with India-Asia convergence rate. Models that aim to explain the causes of the significant Indian plate motion acceleration around 70 Ma, and the subsequent deceleration around 52 Ma predict different scenarios regarding crustal shortening of the Tibetan Plateau, which can be tested by precisely determining the timing of regional shortening events in Tibet. Here we attempt to determine this timing by presenting a high-resolution magnetostratigraphy of a ∼3.5 km thick sedimentary sequence in the syn-contractional Gonjo Basin, east-central Tibet. We successfully isolated the primary remanence as confirmed by positive fold and reversal tests. Correlation to the geomagnetic polarity time scale reveals a 69–41.5 Ma age for the Gonjo Basin sedimentary succession. Average sedimentation rates indicate two episodes of enhanced sediment accumulation rate at 69–64 Ma and 52–48 Ma, which coincide with periods of vertical axis rotation recorded in the basin fill. This coincidence suggests a tectonic cause, which given regional structures we interpret as shortening pulses. Our results are similar to those from basins elsewhere in southern, central and northern Tibet, suggesting plateau-wide, synchronous shortening pulses at ∼69–64 Ma and ∼52–48 Ma. These pulses are synchronous with major acceleration and deceleration of India-Asia convergence rate, suggesting that both the acceleration and deceleration of India-Asia convergence may be associated with enhanced crustal deformation in Tibet, which we use to evaluate previous dynamic models explaining the Indian plate motion changes and India-Asia collision processes. [ABSTRACT FROM AUTHOR]

The poorly dated Gonjo (or Gongjue) Basin, located on the intensely deformed Qiangtang terrane of Tibet where it curves around the Eastern Himalaya Syntaxis, preserves a >3000 m thick terrestrial sedimentary sequence that formed during regional crustal shortening. Here, we present a high-resolution magnetostratigraphy, revealing a 69-41.5 Ma age range, providing robust timing of Late Cretaceous to Eocene shortening in central Tibet. Paleomagnetic declinations moreover show that the Gonjo Basin experienced ~30° clockwise rotation since 52 Ma, consistent with models predicting bending of the Qiangtang terrane associated with shearing along the stable South China Block during Tibetan shortening. Our new age results reveal that the Gonjo Basin correlate with the Hoh Xil Basin in that it underwent a major increase in sedimentation rate, first at 69 Ma, then at 52 Ma, likely indicating enhanced shortening rates during these times. Interestingly, these time periods coincide with the rapid acceleration and deceleration of India-Asia convergence, respectively. We tentatively ascribe the 69 Ma event to increased friction driven by rapid Indian plate subduction resulting from a push force in the Indian ocean. The increase of sedimentation rate at 52 Ma coincides with the dramatically slowdown of India, suggesting that the deceleration corresponds to enhanced friction at the India-Asia plate contact, causing Tibetan shortening. We will discuss subduction scenarios that may explain the delay between ~52 Ma Tibetan shortening and recent India-Asia collision estimates of ~58 Ma. [ABSTRACT FROM AUTHOR]